1. Field of the Invention
This invention relates to data embedding and retrieval algorithms for a color laser printer, and more particularly to a multi-color patch based coding scheme. The invention can be embodied in a method, a program of instructions (e.g., software) and/or an apparatus.
2. Description of the Related Art
Conventionally, most printer-added watermarks have been inserted when the data is in the CMYK domain. (“CMYK” depicts a color space composed of cyan (C), magenta (M), yellow (Y) and black (K)). This is especially true for ink-jet printers in which the device driver has full access to the output dot pattern. However, for color laser printers, a CMYK-domain watermark requires direct access to the CMYK color space, which is deep inside the printer pipeline, as shown in FIG. 1.
For a duplex laser printer, it is not uncommon to apply tone reduction to reduce bleed-thru for plain paper printing. However, this step can adversely affect a watermark added at either the RGB domain or CMYK domain stage of the pipeline. Using tone reduction, even a saturated pixel might not appear on the printout after going through screening. This makes the adage “a dot is a dot” no longer true. On the other hand, if the embedded watermark data is to be printed on the backside of document, the bleed-thru problem must be overcome. An additional constraint to consider is that many laser printer engine manufactures put their own watermark on the Y channel.